1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image by melting a toner, such as a copying machine, a printer, and a MFP, which uses electrophotographic technology, and a curl correcting method of correcting curl generated in a recording medium.
2. Description of the Related Art
Conventionally, an image forming apparatus using electrophotography causes an image carrier to hold charges on its surface by a charging device, forms a latent image on the image carrier by irradiating laser light or light emitting diode (LED) light from a light irradiation device according to image information, and develops the latent image by a toner supplied from a developing device. A toner image on the image carrier is transferred onto a transfer belt by a transfer device and is then transferred from the transfer belt to paper which is a recording medium. The toner image on the paper is fixed on the paper by being heated and pressed by a fixing device.
The fixing device includes a heating roller which heats the toner image, and a pressing roller which presses the paper against the heating roller. The heating roller includes a heating source and is controlled at a constant temperature to melt the toner image. On the other hand, the pressing roller has no heating source. Even if the pressing roller has a heating source, the pressing roller is set to a temperature lower than that of the heating roller so as to reduce power consumption. For this reason, a temperature difference occurs on two sides of the paper which is one example of the recording medium. This temperature difference causes a difference in amount of evaporation of moisture between front and rear sides of the paper, and it causes a difference in shrinkage of front and rear sides. Thereby, curl is generated in the paper. Also, a nip portion bent along a conveying direction may be formed by increasing a contact area of the nip portion so as to sufficiently melt a toner or by making the heating roller and the pressing roller have different hardness so as to make the paper easily separate from a fixing member. In that case, curl replicating the shape of the nip portion is generated in the paper.
The curled paper causes a paper jam in a conveying path after fixation, bulkiness when storing in a discharging unit, or the like. Conventionally, a curl correcting apparatus may be used to correct such curl generated at the time of fixation. As the curl correcting apparatus, a curl correcting apparatus of a roller nip type including an elastic roller and a pressing roller which rotates while forming a curved nip portion by pressing the elastic roller is known. Also, a belt nip type curl correcting apparatus. The belt nip type curl correcting apparatus including a belt which rotates while being wound around a belt support roller, and a pressing roller which rotates while forming a curved nip portion by pressing the belt is known. In such a curl correcting apparatus, both types are configured to correct the paper into a curl-free state by passing the paper after fixation through the curved nip portion between the elastic roller or the belt and the pressing roller and deforming the paper in a direction opposite to a curled direction.
In such a curl correcting apparatus, a different state of curl generated after fixation results, depending on a paper type or a difference in a density of an image formed on the paper environmental humidity and/or the like. For this reason, conventionally, a correction amount of the curl correcting apparatus has been adjusted according to a predetermined parameter, such as the paper type and the image density.
Japanese Patent Application Laid-open No. 2002-316761 discloses an image forming apparatus which aims at performing an appropriate curl correction according to an environmental change or a paper type and is characterized in that a correction amount is calculated according to a surrounding environment or a paper type. More specifically, the technology of Japanese Patent Application Laid-open No. 2002-316761 acquires environmental information around an image forming unit by an environment information input unit, and adjusts a correction amount of paper by a curl correction unit, based on the acquired environmental information. Thereby, even when there is an environmental change, such as a change in ambient temperature or humidity, an appropriate curl correction can be performed. Also, the technology of Japanese Patent Application Laid-open No. 2002-316761 adjusts a correction amount according to a paper type which has been input in advance.
Also, Japanese Patent Application Laid-open No. 2011-081341 performs an exact curl correction by measuring a temperature or humidity near a surface of a transfer material having passed through a pressing portion of a fixing device and predicting an amount of curl formed later. More specifically, in the technology of Japanese Patent Application Laid-open No. 2011-081341, a detection unit which measures the temperature or humidity near the surface of the transfer material having passed through the pressing portion of the fixing device calculates a predicted curl amount from at least one physical quantity related to one or both of the temperature and the humidity, and sets a control value corresponding to the calculated predicted curl amount, so that a correction amount of paper by a curl correction unit is adjusted. Thereby, even when there is a change in a temperature state of the fixing device or a moisture state of the paper, an appropriate curl correction can be performed.
The curl correction device corrects curl by generating residual strain to remove the curl by using a viscoelastic characteristic of the recording medium (for example, paper, film, or the like) which is the transfer material. Therefore, the effect of the curl correction also changes according to the change in the viscoelastic characteristic of the recording medium. The viscoelastic characteristic changes according to the temperature and moisture content of the recording medium (for example, paper, film, or the like). When the fixing device is disposed upstream (previous process) of the curl correction device, the temperature of the heating roller changes according to the operating state of the image forming apparatus, for example, according to whether immediately after start-up or during continuous printing, or whether a color image or a monochromatic image. In particular, the temperature of the pressing roller having no heating source greatly changes. With the change in temperature, the moisture content of the recording medium (for example, paper, film, or the like) at the time of arrival at the curl correction device also changes. In order to accurately perform the curl correction of the recording medium, the correction depending on such a change is required. Therefore, in order to accurately remove the curl of the recording medium, it is necessary to accurately know the temperature and moisture content of the recording medium (for example, paper, film, or the like) before the correction in the curl correction device, that is, at the upstream of the curl correction device. Also, the curl correction according to the viscoelastic characteristic of the recording medium is required.
However, in the conventional technology disclosed in Japanese Patent Application Laid-open No. 2002-316761, since there is no perception that the effect of the curl correction changes according to the temperature and moisture content of the paper, an accurate correction is difficult even though the curl is corrected according to the environmental humidity. That is, in the conventional technology, in some operating situations of the image forming apparatus, the curl suppression effect may be low as compared with the actual curl amount or the suppression effect may be too adjusted so that the recording medium may be reversely curl, and there has been a problem that it is impossible to sufficiently cope with the change in the curl correction effect changing due to the viscoelastic characteristic of the recording medium.
On the other hand, also in the conventional technology disclosed in Japanese Patent Application Laid-open No. 2011-081341, since there is no perception that the effect of the curl correction changes according to the temperature and moisture content of the paper, an accurate correction is difficult even though the curl is corrected by estimating a curl amount from a physical quantity obtained by a detection unit. Therefore, as in the case of Japanese Patent Application Laid-open No. 2002-316761, there has been a problem that it is impossible to sufficiently cope with the change in the curl correction effect changing due to the viscoelastic characteristic of the recording medium.
Also, in order to predict the effect of the curl correction, data to calculate the effect of the curl correction peculiar to each recording medium is required. For example, paper is one example of the recording medium of an existing type but paper available in the market does not always have a constant characteristic. Therefore, the current characteristic may be different from the previous data. Also, if considering coping with new products, data to calculate the effect of the curl correction needs to be updated. Updating data on the image forming apparatus being already running in the market has a problem in that a lot of time and effort is required.
In view of the above problems, there is a need to provide an image forming apparatus and a curl correcting method, which can cope with changes in a temperature state or characteristic of a recording medium after a fixing device according to changes in the operating situation of the image forming apparatus, and realize an appropriate and accurate curl correction.